A charged particle beam apparatus is an apparatus for obtaining a specimen image by use of a signal generated from a specimen by scanning on the specimen with a thinly focused charged particle beam. With a charged particle beam apparatus for use in testing a photomask, and a wafer, to be used in a semiconductor process, in particular, among charged particle beam apparatuses, it is necessary to acquire information such as an image, and so forth, at high speed in order to enhance throughput per unit of time. In focusing of a charged particle beam, use is normally made of a magnetic lens employing an electromagnetic coil. However, in the case of using the magnetic lens, there occurs deterioration in repeatability of a magnetic field produced by the magnetic lens due to an effect of magnetic hysteresis occurring to a magnetic material of the magnetic lens even if current of the electromagnetic coil is set the coincident condition. Accordingly, even if an excitation current of the magnetic lens is set to an excitation condition of the magnetic lens, under which the best image without being defocused should be obtained, an image as acquired will be defocused if the repeatability undergoes deterioration. In consequence, re-adjustment of the magnetic lens is required, resulting in an increase in time length necessary for obtaining the specimen image. It follows therefore that a technology for removing the magnetic hysteresis of the magnetic lens is necessary in order to avoid the increase in the time length as required.
As to a technology for removal of the effect of magnetic hysteresis, Japanese Patent No. 3458481 has disclosed the technology for removing the effect of magnetic hysteresis by unit of executing a sequence of steps for exciting the electromagnetic coil with current at values reversed in polarity from each other, or at a maximum value and a minimum value, which can be set by the apparatus, during stage transfer, and subsequently setting to a desired current of the electromagnetic coil. Meanwhile, in U.S. Pat. No. 5,216,235, there has been disclosed the technology for focusing by varying a position of a specimen plane so as to keep a distance between an objective lens, and the specimen plane constant without changing setting of an electromagnetic coil, as a technology for avoiding a magnetic hysteresis phenomenon of the magnetic lens.